#include<bits/stdc++.h>
#define y1 Y1
#define fi first
#define endl "\n"
#define se second
#define PI acos(-1)
#define int long long
#define pb(x) push_back(x)
#define PII pair<int, int>
#define Yes cout << "Yes\n";
#define No cout << "No\n";
#define YES cout << "YES\n";
#define NO cout << "NO\n";
#define _for(i, a, b) for(int i = a; i <= b; ++i)
#define IOS ios::sync_with_stdio(false),cin.tie(0),cout.tie(0)
using namespace std;

const int N = 2e5 + 10;
int a[N];
int n, m;
string s;

signed main() {
	IOS;
	int ret = 0;
	cin >> s;
	n = s.size();
	if(n % 2) {
		int i = n / 2 - 1; 
		int j = n / 2 + 1;
		//cout << "i = " << i << " j = " << j << endl;
		while(i != -1) {
			if(s[i] != s[j])ret++;
			i--,j++;
		}
	} else {
		int i = n / 2 - 1;
		int j = n / 2;
		while(i != -1) {
			if(s[i] != s[j])ret++;
			i--;
			j++;
		}
	}

	cout << ret << endl;
	return 0;
}
/*
abc
*/

